Recently, in a storage apparatus, data stored in the storage apparatus is managed using a set of files and directories present in the storage apparatus at a point in the past and a snapshot function for realizing recording of the set.
As an application of such a snapshot, for example, Patent Literature 1 discloses a writable snapshot (also referred to as “Writable Snapshot (WSS)” in this specification). In Patent Literature 1, the storage apparatus includes an original snapshot (also referred to as golden image (GI)) and a pool which accumulates a writing request to the original snapshot as a difference (also referred to as difference pool or WSS pool). During reference, the storage apparatus creates a snapshot image after writing from the original snapshot and difference data in the pool to provide a host computer with the writable snapshot (WSS).